As a superconducting cable, a superconducting cable illustrated in FIG. 4 has been proposed. The superconducting cable 100 is configured to accommodate 3 cable cores 10 in a heat insinuating tube 20 (see Patent Document 1 for example).
The cable core 10 includes a former 11, a superconducting conductor layer 12, an insulating layer 15, a superconducting shield layer 16, and a protection layer 18 successively from a center thereof. Normally, former 11 is configured of stranded wire, a pipe member, and/or the like. The conductor layer 12 is configured by spirally winding superconducting wire on former 11 in multiple layers. Representatively, there is used a superconducting wire in a form of tape in which a plurality of pieces of filaments comprising an oxide superconducting material are arranged in a matrix of a silver sheath or the like. For an alternate current cable, an outer superconducting wire in conductor layer 12 reduces an increased density of a current, i.e., non-uniform current distribution, and hence ac loss. Accordingly, each layer may have its superconducting wire wound with a different pitch. The insulating layer 15 is configured by winding insulating paper. The shield layer 16 is configured by spirally winding on the insulating layer 15 a superconducting wire similar to the conductor layer 12. Further, insulating paper or the like is used for the protection layer 18.
The heat insulating tube 20 is a double tube, which includes an inner tube 21 and an outer tube 22, with thermal insulator (not shown) posed therebetween, and is internally vacuumed. A corrosion resistant layer 23 is formed at an outer side of the heat insulating tube 20. Further, the cable is used such that a coolant of liquid nitrogen or the like is introduced into and circulated through a space formed in the former 11 (if it is hollow) or between the inner tube 21 and the core 10 and the insulating layer 15 is impregnated with the coolant.
For such a superconducting cable as described above, it is necessary to ensure a fault-current shunt to prevent for example in an accidental short circuit or the like a fault current from flowing through the superconducting wire and resulting in excessively increased temperature damaging the wire. Accordingly, there has been proposed combining normal conducting material with the superconducting cable's constituent materials. For example, patent document 2 discloses providing a normal conducting metal layer serving as a fault-current shunt at an outer side of a conductor layer formed of superconducting wire. Furthermore, patent document 3 discloses that a stranded wire structure formed of insulated coated wire formed of normal conducting material is used as a material for a core member (or former) to serve as a fault-current shunt.
Patent Document 1: Japanese Patent Laying-open No. 2001-202837 (FIG. 1)
Patent Document 2: Japanese Patent Laying-open No. 2000-67663
Patent Document 3: Japanese Patent Laying-open No. 2001-325838